Description

I'm using a MultipleHiddenInput widget and it seems it only renders the hidden inputs if there is a value bound to the field as opposed to HiddenInput which always renders a hidden input.
I hoped it will render a least one hidden input if the form is not bound.

I don't think what you're describing makes sense. MultipleHiddenInput represents multiple values as a series of hidden inputs. If there aren't any currently selected values,then there aren't any hidden inputs. I'm not sure I see how it could be meaningfully interpreted any other way.